While receptacles and window members are used in packaging LEDs, the window members must be made of materials having high transparency to light emitted by the LED. In the prior art, light-transmissive resins such as epoxy resins, silicone resins, modified acrylic resins and unsaturated polyester resins are used as the window members for LEDs (see Patent Document 1).
As the use of mercury lamps is recently regulated, UV-LEDs capable of emitting short wavelength light, especially UV band light are regarded promising as the replacement. While LEDs can produce light of arbitrary wavelength, LEDs of wavelength for a particular application are developed. For example, wavelength 265 nm in the UV region is known effective for sterilization. UV-LEDs adapted to emit light of wavelength 265 nm are developed for the sterilization application. Even when optical devices of 265 nm are in constant supply, it is difficult to use the optical devices without packages. It is thus required to package the UV-LED while maximizing the light extraction efficiency from the UV-LED.
In packaging of UV-LEDs, borosilicate glass and quartz glass are generally used as the window members (see Patent Document 2). When light-transmissive resins and borosilicate glass are used as the window member, there is the advantage of easy working, but the drawback of low UV transmittance. When quartz glass is used as the window member, there is the advantage of high UV transmittance, but the drawback of difficult working because of the high softening point of quartz glass. For all these materials, mirror finishing of their surface is effective for enhancing the extraction efficiency of light. Once the window member is mirror finished, however, the window member fails to scatter light from an optical device featuring directional emission. For light scattering purpose, a separate member such as diffusing filter is necessary.